Summary of Work:Personality can be defined in terms of enduring individual differences in emotional, interpersonal, experiential, and motivational styles. The five factors of Neuroticism, Extraversion, Openness to Experience, Agreeableness, and Conscientiousness provide a comprehensive taxonomy of personality traits for the description of personality in aging men and women. Collaborative studies have confirmed that the five factors are substantially heritable. An investigation involving 807 twin pairs (490 monozygotic and 317 dizygotic) from the National Merit Twin Study provide additional evidence for the robustness of the heritability of the five factors. Three different measures of the Big Five personality dimensions were developed from the battery of questionnaires used in the National Merit Twin Study: one from trait self-rating scales, one from personality inventory items, and one from an adjective check list. Behavior genetic models showed that, on average, about 55% of individual personality trait variation in this population is associated with genetic differences; 45% with environmental events unique to each twin, situational factors, or genotype-environment interaction; and none to shared environment. Studies of the heritability and molecular genetic bases of personality will continue. - Personality traits, heritability, five-factor model of personality, twin studies, behavior genetics, genetics, environment - Human Subjects